The jibes emerged after Mr McCartney claimed he was the victim of "bullying tactics" by the watchdog.

During Business Questions in the House, Mr McCartney accused Ipsa chief executive Andrew McDonald of using "false innuendo and subterfuge" to smear the name of politicians.

"What do you think that subsequent to me raising these issues of Ipsa's senior management's bullying tactics and threats to try and silence me regarding their spiralling costs?" the Lincoln MP asked Leader of the House Andrew Lansley. "Do you think that their chief executive should show some backbone and meet with me as he refused to do so for over two and a half years, instead of attempting to smear Members of Parliament's names by false innuendo and subterfuge?"

However, responding in a letter afterwards, Mr McDonald denied the bullying allegations and insisted Ipsa was providing good value to the taxpayer.

He went on: "Some of the notes written by you, and attached to your claims, were recently brought to my attention. Having reviewed those notes, I was taken aback by the content, which ranged from the abusive ("you are a pedantic SOAB, aren't you?") through to the offensive ("you're talking s****") and on to the condescending ("simples - or didn't you do geography at Eton and Oxford...").

"We will do all that is reasonable to shield our team from such treatment. This policy and its application in your case have been expressly endorsed by the Board. I had hoped it would be possible to address this behaviour without writing to you formally. In the light of your comments today, I write to ask that you conduct your dealings with us - orally or in writing - in civil terms.

"I do not ask you to like Ipsa or its policies. At the same time, Ipsa's team deserves to be treated in a courteous manner. I ask that, with immediate effect, you desist from correspondence which falls below this standard."

In a statement later, Mr McCartney admitted his messages had been "inappropriate". He said: "I apologise unreservedly to Ipsa for my comments which were inappropriate, and which I regret having made," he said. "I accept that such comments have given cause for offence. You will not see me making similar remarks in the future in respect of Ipsa, which has a difficult and important job to do."